Ionization chamber



Oct. 10, 1961 w. MINOWITZ ET AL 3,004,165

IONIZATION CHAMBER Filed April 25, 1958 Jfee/ l FIG. 3

l I 2 m INVENTORS WILBERT MINOWITZ BY FRED E. ROTH 9/2, him ocu ATTORNEYS United? States, Patent This invention relates toa -ionization chamber'for detecting and measuring betaland gamma radiation. It

has particular reference to a chamber which produces a response to gamma raysthatvaries only slightly as the photon energy is changed. This result is produced by careful selection of the materials used and their disposition within the chamber.

Many forms of ionization chambers have been designed, some for gamma rays, and others for both gamma and beta rays. The beta ray chambers generally have been fragile and easily destroyed because of the thin films required to pass the rays from an external sourceinto the chamber. The present invention employs, a rugged construction which protects the thin film but still presents a sufiicient film area to the incident radiation to achieve sensitivity. L Y

The present invention is responsive to gamma radiation over a widerange of photon energies because materials having a low atomic number are employed in its construction and because the materials are disposed to reduce secondary electrons ejectedfrom the walls. The materials used have about the same effective-atomic number as air and the wall material reacts to photons of various energies in the same manner as the desired air wall.

One of the objects of this invention is to provide an improved ionization chamber which avoids one or more of the disadvantages and limitations of prior art arrangements. I 7

Another object of the invention is to provide an ionization chamber which is sensitive to both beta and gamma radiation. 7

Another object of the invention is to provide an ionization chamber which is sensitive to gamma radiation throughout the range of 80 kilo electron vol-ts to 1.2 million electron volts.

Another object of the invention is to protect the thi beta transparent foil from mechanical injury. 1

The invention includes an ionization chamber for the detection and measurement of beta and gamma radiation. The chamber is sealed and contains an ionizable gas such as air at atmospheric pressure. One of the sides of the envelope is formed with a plurality of perforated metal sheets in combination with a perforated sheet of plastic: and two imperforate sheets of thin metal foil. The other sides ofthe envelope are formed with a metal sheet lined with a sheet of conducting plastic. A single electrode is mounted within the chamber and is connected to a lead-in'conductor.

For a better understanding of the present invention, together with other and further objects thereof, reference is made to the following description taken in connection with the accompanying drawings.

FIG. 1 is a plan view of the chamber showing the perforations for the passage of beta radiation.

FIG. 2 is a partial cross sectional view to an enlarged scale of the chamber shown in FIG. 1, taken along line 2-2 of that figure.

FIG..3 is aside view of the chamber. Referring now to the drawings, the chamber includes an outer envelope of aluminum which may be constructed in two pieces 10, 11, for easy assembly. The chamber may be of any shape but it is convenient to make it in the shape of a disc as shown and use one of the flat sides as the beta transmitting area. This side. includes an outer sheet of steel 12 having perforations 13, a thin film of imperforate steel 14, a second sheet of steel .15 similar to sheet 12 and having similar perforations, a sheet of tin 16 with perforations 17, a sheet of plastic 18 with perforations 20 and an imperforate sheet of aluminum 21. Sheet :18 is preferably a phenol formaldehyde condensation product having an effective atomic number of about 6 and good insulating properties. The chamber side of the aluminum sheet is covered with a layer of carbon (not visible in the drawing) to reduce the emission of secondary electrons from the aluminum at low photon energies. As indicated in the drawing, the perforations in sheets 12, "15, 11-6 and 18, are in alignment so that beta rays incident normal to this surface pass through two thin films, steel .001 inch and aluminum .003 inch. The perforations are large enough so that at least 50 percent of the area is available for beta penetration.

The other flat side of the chamber is formed only with a thin aluminum shell 11 and a liner 2.2 of conducting plastic. The liner may be made in many ways but it has been found that ordinary phenol formaldehyde mixed with finely divided carbon before molding constitutes a low atomic number substance and has sufiicient conductivity to transfer charges from the gas Within the chamber to the shell 11 which is used as one chamber terminal. I

The second chamber electrode is mounted in the chamber space and comprises aflat disc of plastic 23 coated on one or both sides by a conductive coating 24 which may consist of finely divided carbon held by a binder such as sodium silicate or any other convenient conducting film having a low atomic number. Disc 23 is supported by two spaced insulators 25, only one of which appears in the partial view of FIG. 2. Because of its superior insulating qualities the insulators 25 are preferably made of the plastic tetrafiuoroethylene known by the trade name Teflon. Each insulator includes a base portion which fits under the conducting plastic liner 22 and is held in place thereby. One of the insulators, that shown in FIG. 2, encloses aconductor 26' which acts as a lead-in wire and is designed to be connected to an external circuit. The inner end of the conductor passes through the disc 23 and is soldered or otherwise secured to the disc in conductive relationship with coating 24.

The operation of the chamber is similar to the operation of other prior art chambers. When gamma radiation is to be detected, the bottom side or edge is turned toward the radiation source and only gamma rays (or X-rays) penetrate the aluminum shell and the plastic liner to ionize the gas within the chamber and cause conduction between the two terminals. This conduction is determined in the well known manner by placing a high voltage source, such as a battery, in series between the chamber and an indicating instrument, such as a microammeter. The deflection of the instrument not only detects the presence of gamma rays but also measures the amount of such radiation.

When beta radiation is to be measured, the upper flat surface with the perforations is turned toward the source and the additional instrument reading noted. The beta rays will penetratethe films of steel and aluminum and cause ionization in the gas by molecular collision when their energies are greater than .2 electron volts. 1.

When the upper fiat surface containing the perforak tions is turned toward a gamma source; secondary elec; trons are ejected from the steel film and the steel grids. The tin grid is included to increase the absorption of the gamma and X-ray photons. By absorbing these photons ado ares c in the tin, the intensity incident upon the inner surface of the walls, where secondary electrons are ejected, is reduced. The absorption increases sharply in the lower photon energies as a result of the photoelectric absorption process. Because of its relatively high atomic number, tin is a good absorber of photons in the photoelectric region.

In order to prevent the chamber response from falling off too much in the lower energy range (at 80 kev.), the thin aluminum foil is placed on the inside surface of the grid structure. This foil (.003 inch) actually increases the number of secondary electrons produced in this energy region because the atomic number of aluminum is slightly higher than that of air. At higher energies the effect of the aluminum is negligible. The aluminum foil does decrease, to some extent, the sensitivity of the chamber to beta particles.

If a chamber with a grid structure consisting only of the steel grid sandwich, the tin absorber, and the aluminum foil is exposed to gamma radiation normal to the grid, it is found that the response is less than the response normal to the edge. This may be explained as follows When the gamma radiation is incident upon the beta windows, the secondary electrons emanate from the grid portions. When gamma radiation enters the edge, the primary contribution of secondary electrons is from the plastic walls which have a low average atomic number. Because of the high atomic number of the steel and tin grids a greater number of secondary electrons are produced and scattered at sufiicient angles to enter the air volume and produce the added ionization.

To equalize the response in these two directions the phenolic grid element 20 of low atomic number is inserted behind the tin grid to reduce this effect. The result is a response which is uniform in both directions. The phenolic element does not alter the beta radiation sensitivity.

The invention has now been described with reference to a specific and preferred embodiment thereof. Obviously various changes particularly in the low atomic materials employed in the construction of the ionization chamber, could be made without departing from the spirit of the invention or the scope of the accompanying claims. For example, instead of aluminum of atomic number 14, a metal of lower atomic number such as magnesium or beryllium could be used; instead of carbon of atomic number 6, a conductor of lower atomic number such as beryllium or lithium could be used; and a conductive plastic having an effective atomic number less than that of phenol formaldehyde could be used. Other variations will occur to those skilled in the art.

We claim:

1. An ionization chamber for the detection and measurement of beta and gamma radiation comprising: a sealed envelope containing an ionizable gas, one side of said envelope including a plurality of perforated metal sheets in combination with a perforated sheet of plastic and two imperforate sheets of metal for providing uniform gas ionization due to gamma radiation for all orientations of the chamber with respect to the radiation direction, said perforations arranged in alignment; the other sides of said envelope comprising a sheet of metal lined with a sheet of conducting plastic; an electrode mounted within said envelope and connected to a lead-in conductor for connection to an external circuit.

2. An ionization chamber for the detection and measurement of beta and gamma radiation comprising: a sealed envelope containing an ionizable gas, one side of said envelope including a plurality of perforated metal sheets in combination with a perforated sheet of plastic and two imperforate sheets of metal for providing uniform gas ionization due to gamma radiation for all orientations of the chamber with respect to the radiation direction, said perforations arranged in alignment; the other sides of said envelope comprising a sheet of metal lined with a sheet of conducting plastic; an electrode mounted said envelope and connected to a lead-in conductor for connection to an external circuit, said electrode comprising a sheet of plastic covered with a conducting film.

3. An ionization chamber for the detection and measurement of beta and gamma radiation comprising: a sealed envelope containing air at atmospheric pressure, one side of said envelope including a plurality of perforated metal sheets in combination with a perforated sheet of plastic and two imperforate sheets of metal for providing uniform gas ionization due-to gamma radiation for all orientations of the chamber with respect to the radiation direction, said perforations arranged in alignment for the passage of beta radiation; the other sides of said envelope comprising a sheet of metal lined with a sheet of plastic having interspaced carbon particles to render the plastic sheet conductive; an electrode mounted within said envelope and connected to a lead-in condue tor for connection to an external circuit, said electrode comprising a sheet of plastic covered with a conducting film.

4. An ionization chamber for the detection and measurement of beta and gamma radiation comprising: a sealed envelope containing an ionizable gas, one side of said envelope including a perforated steel sheet, a perfo rated tin sheet, a perforated plastic sheet, and'twoimperforate sheets of metal, said perforations arranged in alignment; the other sides of said envelope comprising a sheet of metal lined with a sheet of conducting plastic; an electrode mounted within said envelope and connected to a lead-in conductor for connection to an external circuit, said electrode comprising a sheet of plastic covered with a conducting film.

5. An ionization chamber for the detection and measurement of beta and gamma radiation comprising: a sealed envelope containing air at atmospheric pressure, one side of said envelope including a perforated steel sheet, a perforated tin sheet, a perforated plastic sheet, and two imp-erforate sheets of metal, said perforations arranged in alignment; the other sides of said envelope comprising a sheet of metal lined with a sheet of conducting plastic; an electrode mounted within said envelope and connected to a lead-in conductor for connection to an external circuit, said electrode comprising a sheet of plastic covered with a conducting film of carbon.

6. An ionization chamber for the detection and measurement of beta and gamma radiation comprising: a sealed envelope in the form of a fiat disc containing air at atmospheric pressure, one of said flat surfaces of the disc including a perforated steel sheet, a perforated tin sheet, a perforated plastic sheet, and two imperforate sheets of metal, said perforations arranged in alignment; the other flat side and circumference of said disc comprising a sheet of metal lined with a sheet of conducting plastic; an electrode mounted within said envelope and connected to a lead-in conductor for connection to an external circuit, said electrode comprising a sheet of plastic covered with a conducting film.

7. An ionization chamber for the detection and meas: urement of beta and gamma radiation comprising: a sealed envelope in the form of a flat disc containing air at atmospheric pressure, one of said flat surfaces of the disc including a perforated steel sheet, a perforated tin sheet, a perforated plastic sheet, an imperforate sheet of steel and an imperforate sheet of aluminum, said sheets arranged in adjoining relationship with the perforations in alignment, the other fiat side and circumference of said disc comprising a sheet of aluminum lined with a sheet of conducting plastic; an electrode mounted within said envelope and connected to a lead-in conductor for connection to an external circuit, said electrode comprising a sheet of plastic covered with a film of carbon.

8. An ionization chamber for the detection and measurement of beta and gamma radiation comprising: a sealed envelope containing an ionizable gas, one side of said envelope including a plurality of perforated sheets with their perforations in alignment, said side also including an imperforate metal film having an atomic number not greater than 14; the other sides of said envelope comprising a metallic sheet having an atomic number not greater than 14, lined with a conducting plastic having an effective atomic number not greater than 6; and an electrode mounted within said envelope and connected to a lead-in conductor for connection to an external circuit, said electrode comprising a sheet of plastic covered with a film of a conductor having an atomic number not greater than 6.

References Cited in the file of this patent UNITED STATES PATENTS Smoluehowski Mar. 29, 1949 Raper May 6, 1952 Mandeville Nov. 11, 1952 Borkowski May 12, 1953 Lynch July 6, 1954 Bell Feb. 28, 1956 Stellmacher Sept. 10, 1957 Birkhofi Feb. 24, 1959 Hermsen Aug. 11, 1959 

1. AN IONIZATION CHAMBER F OR THE DETECTION AND MEASUREMENT OF BETA AND GAMMA RADIATION COMPRISING: A SEALED ENVELOPE CONTAINING AN IONIZABLE GAS, ONE SIDE OF SAID ENVELOPE INCLUDING A PLURALITY OF PERFORATED METAL SHEETS IN COMBINATION WITH A PERFORATED SHEET OF PLASTIC AND TWO IMPERFORATE SHEETS OF METAL FOR PROVIDING UNIFORM GAS IONIZATION DUE TO GAMMA RADIATION FOR ALL ORIENTATIONS OF THE CHAMBER WITH RESPECT TO THE RADIATION DIRECTION, SAID PERFORATIONS ARRANGED IN ALIGNMENT, THE OTHER SIDES OF SAID ENVELOPE COMPRISING A SHEET OF METAL LINED WITH A SHEET OF CONDUCTING PLASTIC, AN ELECTRODE MOUNTED WITHIN SAID ENVELOPE AND CONNECTED TO A LEAD-IN CONDUCTOR FOR CONNECTION TO AN EXTERNAL CIRCUIT. 